Section 69. The commissioner may, after the lapse of three years from the date of their receipt, or three years after the end of the taxable period to which they relate, whichever occurs earlier, destroy copies, returns and statements made under sections eighty-three and eighty-six of chapter fifty-nine and any returns and documents filed with him for the purposes of taxation; provided however, that such copies, returns, statements or documents may be destroyed at any time if microfilm or other copies thereof are retained.